1. Field of the Invention
The invention relates to a pixel driving device, and more particularly to a pixel driving device for a liquid crystal display and a method for driving the same.
2. Description of the Related Art
Display technology has seen great advances. Conventional cathode ray tubes (CRTs) have been gradually superseded by liquid crystal display (LCD) in the high-end display market. CRTs have some major drawbacks such as large size and high radiation emissions while LCD monitors have advantages of no radiation emissions, low power consumption, and lightweight.
FIG. 1 is a schematic diagram showing a pixel driving device for a pixel in a conventional thin film transistor liquid crystal display (TFT-LCD) panel. The LCD panel includes a plurality of pixels arranged as a matrix. Each pixel has a pixel driving device for driving liquid crystal molecules of the pixel. The pixel driving device includes a thin film transistor (TFT) having a gate electrode coupled to a scan line SN and a source electrode coupled to a data line DM. The pixel driving device further includes a pixel capacitor CLC and a storage capacitor CST wherein the storage capacitor CST stores charges to hold a voltage across the pixel capacitor CLC, thus keeping the gray scale of the pixel stable. A drain electrode of the TFT is coupled to the pixel capacitor CLC and the storage capacitor CST. The storage capacitor CST and the pixel capacitor CLC are connected in parallel to a common line LCOM. The connection for the storage capacitor CST is called a conventional “CST on common” mode.
When the LCD displays frames, a drive circuit sequentially enables each scan line and turns on the TFTs of each row of pixels on the panel. Meanwhile, the drive circuit sequentially applies pixel voltages Vp from the data line corresponding to each of the pixels. The pixel voltage Vp is applied to the pixel capacitor CLC and the storage capacitor CST. Meanwhile, the common line also provides a common voltage. The capacitor voltages of the pixel capacitor CLC and the storage capacitor CST are determined according to the voltage difference of the common voltage and the pixel voltage Vp. The pixel capacitor voltage difference is utilized to drive the liquid crystal molecules of the pixel giving the pixel a desired gray scale value while the storage capacitor voltage difference is utilized to hold the desired gray scale stable. Since the storage capacitor CST and the pixel capacitor CLC are connected in parallel to the common line LCOM, the values of the capacitor voltages of the pixel capacitor CLC and the storage capacitor CST are the same.
FIGS. 2A to 2B illustrate the arrangement of the liquid crystal molecules in a twisted nematic (TN) mode liquid crystal panel with and without the pixel voltage Vp applied, respectively. In FIGS. 2A and 2B, the arrows show the indicating directions of a front-plate alignment film 204 and a rear-plate alignment film 202 in the TN mode liquid crystal panel. In particular, the indicating directions of the front-plate alignment film 204 and the rear-plate alignment film 202 are perpendicular to each other. The directions of long axes of the liquid crystal molecules 200 close to the alignment films 202 and 204 are substantially parallel to the indicating directions of the alignment films 202 and 204, respectively. When no pixel voltage Vp is applied, the liquid crystal molecules 200 gradually twist until the uppermost layer close to the front-plate alignment film 204 is at a 90-degree angle to the rear-plate alignment film 202, as shown in FIG. 2A. Under these conditions, the liquid crystal molecules 200 possess high light transmission rates, and the pixel's brightness reaches a maximum. FIG. 2B shows that when the proper pixel voltage Vp is applied, the liquid crystal molecules 200 are rotated to be in parallel with the direction of the electric field. In this case, the liquid crystal molecules 200 possess low light transmission rate, and the brightness of the pixel is reduced.
During the manufacture of the panel, the gate electrode of the TFT and the lower electrode of the storage capacitor CST for a pixel are formed in one manufacturing step. In addition, the drain and source electrodes of the TFT, and the upper electrode of the storage capacitor CST for the pixel are all formed in another manufacturing step. For the sake of description, the gate electrode of the TFT and the lower electrode of the storage capacitor CST are referred to as a first metal layer M1, while the drain and source electrodes of the TFT and the upper electrode of the storage capacitor CST are referred to as a second metal layer M2. A silicon nitride (SiNx) layer is provided between the lower electrode and the upper electrode of the storage capacitor CST to serve as a dielectric material between the two plates of the storage capacitor CST.
Due to the possibility for error when manufacturing the panels, the silicon nitride layer between the lower electrode and the upper electrode of the storage capacitor CST may be doped with impurities or other substances, or voids may be formed in the silicon nitride layer. If this occurs, the first metal layer and the second metal layer are short-circuited. If the two metal layers short-circuit, the electrical potentials of the lower and upper electrodes of the storage capacitor CST for the pixel are equal regardless of the magnitude of pixel voltage Vp applied to the pixel. The voltage difference between the lower and upper electrodes of the pixel of the liquid crystal panel would be zero. The pixel in this case is faulty. In a TN mode liquid crystal panel, when the above-mentioned problem occurs in the storage capacitor of a pixel, the faulty pixel always displays its brightness regardless of the applied pixel voltage Vp, and causes a bright spot, especially, for a normally white TN mode liquid crystal panel. When the liquid crystal panel has a bright spot, the display quality of the liquid crystal panel is seriously degraded and customers are not willing to buy these products.